The present invention relates to an electric power steering apparatus that detects torque applied to a steering shaft of a vehicle for steering operation, and drives and controls a steering force assisting motor based on the detected torque.
An electric power steering apparatus, which assists steering by driving a steering force assisting motor based on the detection result of torque applied to the steering wheel for steering operation and transmitting a turning force from the motor to the steering mechanism, has the advantage of more easily controlling the assisting force characteristics according to a running state such as the vehicle speed and the frequency of steering, compared to a hydraulic power steering apparatus using a hydraulic actuator as the source of the steering assisting force. In recent years, the range of its application is being expanded.
Such an electric power steering apparatus needs to detect steering torque applied to the steering wheel. In general, the detection is accomplished by detecting rotational angles at two positions distant in the axial direction of the steering shaft connecting the steering wheel and the steering mechanism. In other words, since the steering shaft has torsion due to the action of the torque applied to the steering wheel during the steering operation and the torsion corresponds to the difference between the rotational angles detected at the two positions, it is possible to calculate the torque based on this difference.
The steering shaft is constructed by connecting an input shaft on the steering wheel side and an output shaft on the steering mechanism side through a torsion bar of a small diameter so that a large rotational angle difference is produced between the input shaft and the output shaft with the torsion of the torsion bar. By detecting the rotational angles at the connection parts of the input shaft and output shaft, the calculation accuracy of the torque can be improved.
The applicant of this application proposed, in Japanese Patent Application Laid-Open No. 2000-352502, etc., a torque detecting apparatus suitable for detecting torque in an electric power steering apparatus. This torque detecting apparatus has sensor units provided respectively at the connection parts of the input shaft and output shaft. The sensor units comprise magnetic targets which are provided in a circumferential direction of an object rotational axis and tilted approximately equally with respect to the axial direction; and magnetic sensors (MR sensors) which are provided outside the magnetic targets to face the same and generate outputs varying with the passage of the targets.
In this construction, the magnetic sensor in each of the sensor units outputs a voltage that changes linearly with an inclination corresponding to the tilt of the targets arranged on the circumference of the input shaft and output shaft as each target passes by. Accordingly, the rotational angles of the input shaft and output shaft can be detected in a non-contact manner based on the output voltages from the corresponding magnetic sensors, and the steering torque applied to the input shaft with the operation of the steering wheel is calculated based on the difference between the rotational angles of the input shaft and output shaft, which is given as the difference between the outputs of the magnetic sensors corresponding to the input shaft and the output shaft.
By the way, for the control of the steering force assisting motor in the electric power steering apparatus, it is necessary to detect the magnitude of the torque applied to the steering wheel and also the direction of the torque, namely, whether the steering is performed to the left or right direction. The torque detecting apparatus constructed as described above compares the rotational angle of the input shaft and that of the output shaft, and judges that the torque is applied in the direction in which the input shaft precedes the output shaft.
In addition to the proper input torque (steering torque) applied from the input shaft on the steering wheel side according to the steering operation, a reaction force from the road surface applied to steered wheels acts on the steering shaft of a running vehicle as an input torque from the output shaft on the steering mechanism side (hereinafter referred to as the “inverse input torque”). For example, during straight driving without turning the steering wheel, if the vehicle runs on a rough road surface and large inverse input torque is applied, the inverse input torque may be detected by the torque detecting apparatus constructed as described above.
However, the inverse input torque as described above is applied such that the rotation of the output shaft side precedes that of the input shaft side, while the proper steering torque is applied such that the rotation of the input shaft side precedes that of the output shaft side. Consequently, in judging the torque direction based on the detection result by the torque detecting apparatus, the inverse input torque is judged to be steering torque applied in the direction in which the steering wheel is energized by the action of the reaction force from the road surface, which is a cause of the inverse input torque. When the steering force assisting motor is driven according to the torque detection, a driving force of the motor is applied to the steering mechanism in the same direction as the inverse input torque, and the driver feels such that the primary inverse input torque is promoted. The conventional art thus has a problem of causing a deterioration of steering feeling.
As a technique related to such a problem, the applicant of the present invention proposed, in Japanese Patent Application Laid-Open No. 2002-29431, “an electric power steering apparatus” comprising means for detecting rotational angles of a steering shaft, which connects a steering mechanism and a steering wheel of a vehicle, at different positions in the axial direction of the steering shaft and calculating the angular velocities of the steering shaft at the respective positions based on the detection results; a speed sensor for detecting the running speed of the vehicle; and control means for reversing the driving direction of a motor, which is determined by the result of calculating steering torque, when a predetermined correlation is established between the calculated angular acceleration and the detected vehicle speed.
Moreover, the applicant of the present application proposed, in Japanese Patent Application Laid-Open No. 10-258756(1998), “a steering apparatus for a vehicle” that is capable of switching a steering mode between an automatic steering mode in which an actuator generates a steering force based on an instruction signal from a control unit and a normal steering mode in which a driver generates a steering force, comprises: means for calculating values corresponding to the rotational angle on the steering wheel side of the steering shaft in a time series; and means for calculating values corresponding to the rotational angle on the wheel side of the steering shaft in a time series, judges that the applied steering force was applied by the driver, if a change in the rotational angle on the steering wheel side precedes a change in the rotational angle on the wheel side, judges that the applied steering force was applied to the vehicle from the outside, if a change in the rotational angle on the wheel side precedes a change in the rotational angle on the steering wheel side, and switches to the normal steering mode from the automatic steering mode when the value corresponding to the applied steering force is equal to or higher than a preset value.